Dissimilatory reduction of metal and nonmetal elements is a process in which metal and nonmetal elements are reduced as the terminal electron acceptor during microbial respiration. The research on the dissimilatory reduction of metal and nonmetal elements has received great attention from scholars in various countries. This dissimilatory reduction can not only reduce or eliminate harm to human being's health brought by toxic metal and nonmetal ions in the environment and effectively recover all kinds of precious metals in polymetallic ores and tailings, but also produce nanomaterials and catalysts via artificial approach. The study on the bio-reduction of selenium, tellurium and other non-metal elements has become a hot spot in the international material field.
The study on the metal dissimilatory reduction traces back to using microorganisms to reduce Fe (III) and Mn (IV). It has been found that microorganisms can utilize organic acids or hydrogen as electron donor to reduce Fe (III) and Mn (IV) to Fe (II) and Mn (II) respectively. In the past decade, scholars have also discovered that chromium (VI), Vanadium (V), Cobalt (III), Palladium (II), Rhodium (III), Europium (III) and some radioactive elements, such as Uranium (VI), Plutonium (VI), Neptunium (V), Technetium (VII), and other high-valence ions or oxides, which act as electron donors, can be reduced to the low-valence ions or oxides by microorganisms oxidizing organic acids or hydrogen. In addition, some ions such as Palladium, Mercury, Rhodium and Europium can be directly reduced to elementary substances by microorganisms.
In the existing published patent literature, two U.S. patents (U.S. Pat. No. 5,569,596 and U.S. Pat. No. 5,739,028) respectively describe a chromium (VI) resistant strain Shewanella alga reducing chromium (VI) to form insoluble chromium (III) sediment under anaerobic conditions and a method of removing the sediment from pollutants. A Chinese patent (patent number: CN93106616.6) describes strains of anaerobic bacteria such as Fusobacterium nucleatum reducing chromium (VI) to generate insoluble chromium (III) precipitation, and the usage to remove chromium from some heavy metal wastewater such as electroplating wastewater. Two U.S. patents (U.S. Pat. No. 5,055,130 and U.S. Pat. No. 5,283,192) describe a strain of Bacillus polymyxa treating the silver manganese mine and a method of promoting recovery of manganese and silver by reducing of Mn (IV) oxide to soluble Mn (II) ions. Moreover, a U.S. patent (U.S. Pat. No. 6,218,171) reports the method of non-growing cells of two Shewanelia bacteria (Shewanelia putrifacians and Shewanelia alga) reducing the radioactive element Tc (VII) to Tc (IV) under the anaerobic conditions. In addition, there are two U.S. patents (U.S. Pat. No. 5,352,608 and U.S. Pat. No. 5,804,424) which respectively cover a photosynthetic bacteria (Rhodobacter sphaeroides) reducing the rhodium (III) and Eu (III) oxide into elementary substances under anaerobic conditions and depositing on the cell membrane. Lastly, a Chinese patent (patent No. ZL 99815312.5) describes some reducing bacteria such as Desulfovibrio sp., Pseudomonas sp., Shewanella sp., which are used in heavy metal wastewater treatment and reduce iron (III) and manganese (IV) to their low state, respectively.
In addition to patent documents, there are many articles which report the research of the microbial reduction of metals, most of which focus on the earliest discovery of bioreduction of iron (III) and manganese (IV). D. R. Lovely who has made an outstanding contribution in this field, published three articles about the biological reduction of iron (III) in Nature (Anaerobic production of magnetite by a dissimilatory iron-reducing microorganism, Nature, 330, 252-254, (1987); Oxidation of aromatic contaminants coupled to microbial iron reduction, Nature, 339, 298-300, (1989); Geobacter metallireducens accesses insoluble Fe(III) oxide by chemotaxis, Nature, 416, 767-769, (2002)). Moreover, many articles relate to the application of Shewanella sp. and Geobacter sp. in the field of metal reduction. Chromium (VI) is a hazardous substance in the environment, whose biological reduction has also been paid attention by scholars. More literature reports the sulfate-reducing bacteria on Cr (VI) reduction and the formation of insoluble chromium (III) compounds, including literature written by some Chinese scholars such as Fude Li and Jidong Gu (Ji-Dong Gu, International Biodeterioration & Biodegradation, 59, 8-15, (2007); Fude Li et al., Study on Reduction of Hexavalent Chromium(VI) by Sulfate-reducing Bacteria, 1993, Environmental Science, 14(6): 1-4). Hillol and Sarah reported the chromium (VI) reduction by the Shewanella alga and Shewanella oneidensis MR-1, respectively (Sarah S. Middleton, et al., Cometabolism of Cr(VI) by Shewanella oneidensis MR-1 Produces Cell-Associated Reduced Chromium and Inhibits Growth. Biotechnology and Bioengineering 83(6), 627-637, (2003). Hillol Guha, Biogeochemical influence on transport of chromium in manganese sediments: experimental and modeling approaches, Journal of Contaminant Hydrology, 70, 1-36, (2004)). Furthermore, Vanadium (V) as a high-valence metal has been more studied in recent years. Researchers have found that such bacteria as Geobacter sp., Shewanella sp. and Pseudomonas sp. have the ability of reducing vanadium (V) to the insoluble vanadium (IV) compounds (Judith M. Myers et al., Vanadium (V) Reduction by Shewanella oneidensis MR-1 Requires Menaquinone and Cytochromes from the Cytoplasmic and Outer Membranes, Appl. Environ. Microbial., 70(3), 1405-1412, (2004); Irene Ortiz-Bernad et al., Vanadium Respiration by Geobacter metallireducens: Novel Strategy for In Situ Removal of Vanadium from Groundwater. Appl. Environ. Microbial., 70(5), 3091-3095, (2004); A. N. Antipov et al., Vanadate Reduction by Molybdenum-Free Dissimilatory Nitrate Reductases from Vanadate-Reducing Bacteria. IUBMB Life, 50(1), 39-42, (2007)). Moreover, as for radioactive elements uranium (VI), because of its danger in the environment, biological reduction has become an important tool to eliminate the hazard. Two papers in Nature are both about the study on bioreduction of uranium (VI). As early as in 1991, D. R. Lovely published an article in Nature which reported that the use of iron (III)-reducing bacteria (Alteromonas putrefaciens) could reduce the U(VI) to insoluble uranium (IV) and the rate of reduction was significantly faster than that of non-biological reduction process. Some scholars have reported the research on the use of Shewanella sp. and Geobacter sp. to reduce other metal elements, which are high valence ions, such as Cobalt (III) (Caccavo Jr, F. et al., Geobacter sulfurreducens sp. nov., a hydrogen and acetate-oxidizing dissimilatory metal reducing microorganism, Appl. Environ. Microbiol. 60, 3752-3759, (1994)), Neptunium (V) (Lloyd, J. R., et al., Biological reduction and removal of pentavalent Np by the concerted action of two microorganisms, Environ. Sci. Technol. 34, 1297-1301, (2000)), Tc (VII) (Lloyd, J. R. and Macaskie, L. E. A novel phosphorImager based technique for monitoring the microbial reduction of technetium, Appl. Environ. Microbiol. 62, 578-582, (1996)), whose reduction products are low valence Cobalt (II), Neptunium (IV) and Tc (IV), respectively.
With the exception of the above metal elements, the bioreduction of some precious metals such as gold, silver, platinum, and palladium has been received a great attention by scholars. Some research results have been applied to precious metals smelting and processing of metal catalysts. Unlike other metal elements, owing to the inertia of the precious metal, most bioreduction products of them are elementary substances. Furthermore, microorganisms that participate in reduction show a biological diversity, among which sulfate-reducing bacteria, Shewanella, and Bacillus, Pseudomonas, Enterobacteria, Corynebacterium etc. are involved in the process of reduction. Chinese scholar YueYing Liu has done a large number of researches not only on reducing some precious metals ions such as gold, platinum, palladium to a simple substance, but also in the field of Palladium catalysts (Studies on Biosorption of Au(Au3+) by Bacillus Megaterium D01, Acta Microbiologica Sinica, 2000, 40(4):425-429). What's more, Wiatrowski H. A. uses Shewanella sp. and Geobacter sp. to reduce mercury (II) to mercury (0) directly through microbial reduction (Novel reduction of mercury (II) by mercury-sensitive dissimilatory metal reducing bacteria. Environ Sci Technol, 40(21), 6690-6, (2006)).
To sum up, with the exception of a small number of inert precious metal elements, the vast majority of metal elements in biological reduction are limited to transformation from high-valence to low and most completed under anaerobic conditions, while it is rare to be reported that metal ions are reduced directly into elementary state.
While from the standard electrode potential (in acid solution) in electrode reaction of the metal element, we can see that the precious metal elements gold (I), silver (I) and Pd (II) are reduced to their elementary substances with higher electrode potential, 1.68 (v), 0.80 (v) and 0.99 (v) respectively. In addition, the precious metal element platinum (IV) can be reduced by microorganism to the low valence platinum (II) or elemental platinum (0) by microorganism. The potential of two electrode of reduction reaction are 0.68 (v) and 0.73 (v) respectively. Apart from precious metals, mercury can be reduced to elementary substance by the microorganisms, and from the electrode potential, we can see that the electrode potential of Mercury (II) to reduce remains high, reaching 0.85 (v). As for the general metal elements such as iron (III), manganese (IV), chromium (VI), Vanadium (V), cobalt (III), uranium (VI) and plutonium (VI), when they are reduced from their high valence to the corresponding low state, i.e. iron (II), Mn (II), chromium (III), Vanadium (IV), cobalt (II), uranium (IV) and plutonium (IV), their electrode potential are 0.77(v), 1.23 (v), 1.33 (v), 1.00 (v), 1.82 (v), 0.62 (v) and 0.97(v), respectively.
The analysis of electrode potential of metal elements whose reduction involves microorganisms shows that the existing bioreduction process has a higher electrode potential, between 0.6 (v) and 1.8 (v). While metal elements with lower valence, such as iron (II), Mn (II), chromium (III), vanadium (II), Co (II), Uranium (III) and plutonium (IV) are further reduced to their zero valence substances, and their potentials in the electrode reaction are −0.44 (v), −1.18 (v), −0.74 (v), −1.18 (v), −0.28 (v), −1.8 (v) and −2.42 (v) respectively. So far, electrode reaction phenomenon of reducing such low electrode potential metal elements by microbes has not yet reported in the publications.
As far as the reduction of non-metal is concerned, the oxidation state of common non-metallic elements such as sulfate and nitrate are reduced to sulfide or nitrogen, nitrous oxide and other products by the sulfate-reducing bacteria or nitrate-reducing bacteria (denitrifying bacteria). In addition, in the area of sulfate reduction and nitrate reduction there has been too many literature and monographs published. As we know, a large number of microorganisms in the nature are involved in the biological reduction process of nitrogen oxide and sulfur oxide.
Moreover, the scholars have given enough attention to the biological reduction of some non-metallic elements such as arsenic, selenium and tellurium because of the health risks brought by an excess of them in the environment. Two U.S. patents (U.S. Pat. No. 5,352,608 and U.S. Pat. No. 5,804,424) describe under photosynthetic and heterotrophic conditions, two photosynthetic bacteria (Rhodobacter sphaeroides and Rhodobacter capsulatus) being able to reduce selenite and tellurite to their element (selenium and tellurium). Another two U.S. patents (U.S. Pat. No. 5,009,786 and U.S. Pat. No. 5,271,831) describe how to use microbial anoxic system to treat selenate wastewater, in which microorganisms are able to make use of organic acids and hydrogen as electron donor for the direct reduction of selenate, and the residual selenite can be reduced into single-selenium(0) by hydrogen sulfide created by microbes.
To date, there are a lot of reports relating to selenium and tellurium reduction. The early studies mainly concentrated on the resistance of microorganisms to selenium and tellurium. Since the mid-1990s, studies using photosynthetic bacteria to reduce tellurite and selenite to Te (0) and Se (0) have begun to be reported. V. Yurkov and A. Yamada respectively reported that photosynthetic bacteria (Roseococcus thiosulfatophilus, Erythromicrobium ezovicum, Rhodobacter sphaeroides) could reduce tellurite and selenite to Se (0) and Te (0) both under aerobic and anaerobic conditions. In addition, a Chinese scholar Dongliang Wang (Screening and identification of a photosynthetic bacterium reducing selenite to red elemental selenium, Acta Microbiologica Sinica, 2007, 47(1): 41-47) and Janine Kessi (Enzymic systems proposed to be involved in the dissimilatory reduction of selenite in the purple non-sulfur bacteria Rhodospirillum rubrum and Rhodobacter capsulatus. Microbiology, 152, 731-743, (2006)) separately reported the screening technology of a strain of selenite-reducing photosynthetic bacteria Rhodobacter azotoformans as well as the enzyme systems involved in the processes that Rhodospirillum rubrum and Rhodobacter capsulatus reduced selenite to Se (0). Besides photosynthetic bacteria, researchers also studied the reduction of selenite and tellurite based on the mechanisms of denitrifying bacteria reducing nitrate: Sridhar Viamajala investigated the reduction of selenitle when denitrifying bacteria were cultured in batch and reactor (Selenite reduction by a denitrifying culture: batch- and packed-bed reactor studies. Appl Microbiol Biotechnol, 71, 953-962, (2006)); MONIQUE SABATY et al. studied on the reduction of selenite and tellurite through the combination of periplasm and membrane of denitrifying bacteria with denitrifying enzyme (Characterization of the Reduction of Selenate and Tellurite by Nitrate Reductases., Appl. Environ. Microbial., 67(11), 5122-5126, (2001)); Michihiko Ike (Selenate Reduction By Bacteria Isolated From Aquatic Environment Free From Selenium Contamination. Wat. Res. 34(11), 3019-3025, (2000)) and J. M. Rajwade (Bioreduction of tellurite to elemental tellurium by Pseudomonasmendocina MCM B-180 and its practical application. Hydrometallurgy, 71, 243-248, (2003)) reduced selenate, selenite and tellurite to Se(0) and Te(0) with Pseudomonas spp (Pseudomonas stutzeri, Pseudomonas uorescens and Pseudomonas mendocina), respectively; Agnieszka Klonowska et al. studied the reduction of selenite and tellurate, utilizing metal-reducing bacteria under anaerobic conditions and found a large number of elemental selenium and tellurium precipitated outside the membrane in the form of nanoparticles; Chinese scholar Jidong Gu et al. (Chromate reduction by Bacillus megaterium TKW3 isolated from marine sediments, World Journal of Microbiology & Biotechnology, 21, 213-219, (2005)) and Ruiping Li et al. (Sodium selenite reduction to elemental selenium by Bacillus HBS4, Acta Petrologica Et Mineralogica, 598-603, 24(6), 2005) used Bacillus spp. to reduce selenite and also got elemental selenium. Studies on arsenic reduction are comparatively less. It has been found that arsenic (V) could be reduced to soluble but more toxic V(III) by SR-bacteria and pseudomonas under anaerobic conditions.
As a very important photoelectric conversion element, non-metal element silicon plays an important role in the field of solar cells and semiconductor materials as well as other photoelectric materials. However, we have not found any report about bio-reduction of silicon in the literature.
In conclusion, it seems that the study on the bio-reduction of rare nonmetal elements such as selenium and tellurium has been a hot point all over the world with the development of material science and made considerable progress, in addition to studies on the bio-reduction of conventional nonmetal elements such as nitrate and sulphate as well as their general application. As shown in the existing literature, elements mentioned above are the only ones that could be bio-reduced while there is no report concerning other elements found in the field of non-metal elements reduction.
The standard electrode potential of nonmetallic elements electrode reactions in acid solution shows that the electrode potentials are 0.96 (v) and 0.20 (v) respectively when nitrate and sulphate are reduced from high-valence (nitrogen(V), sulphur(VI)) to low-valence state (nitrogen (II), sulphur(IV)), while the electrode potentials of nitrogen (II) reduced through nitrogen (I) to nitrogen (0) are both higher (1.59 (v) and 1.77 (v), respectively), and, sulphur (IV) to sulphur(0) is 0.45 (v). From recent studies on bioreduction of selenium and tellurium, we can see that the electrode potentials of selenium and tellurium are 0.74 (v) and 0.56 (v) respectively when they are reduced from high-valence selenium (IV) and tellurium (IV)) to low valence (selenium (0) and tellurium (0)). Even that arsenic is reduced from arsenic (V) to arsenic (III), the electrode potential of arsenic can reach 0.56 (v).
Seeing from the above-mentioned elements, most of them can get a high potential (0.40v˜1.77v) when they are reduced to simple substances, besides sulphur which is relatively low (0.20v) when it is reduced from sulphur (VI) to sulphur (IV). As for silicon (IV), the electrode potential of it can reach to −0.86 (v) when it is reduced to silicon (0), which may be the important reason for the high-valence silicon to be bio-reduced difficulty.
Treatment of heavy metal pollution has become an important environmental problem in recent years. Traditional methods to treat heavy metal pollution include chemical precipitation, electrolysis, ion exchange and physical adsorption etc. Biosorption has been a rapid-developing emerging heavy metal treatment technology in recent 10 years compared to those traditional methods. Biosorption is an effective approach to adsorb and recover metal ions by ion exchange, surface complexation, redox and electrostatic adsorption by using live or dead cells, which has advantages of high efficiency at low concentration, high adsorption capacity, selectivity, easy operation. Biosorption of heavy metals related research has been the focus of international environment field. Bacteria, alga, fungi, and some of their components have been successfully applied in removal of heavy metal ions from streams. Among these organisms, because of its high specific surface area, the bacteria caused widespread concern.
The important progress of biosorption firstly comes into the research of sulfate reducing bacteria and its application in biology treatment of heavy metal wastewater. Sulfate reducing bacteria and their metabolites (soluble sulfides) play an important role in the adsorption of metal ions, such as chromium, cadmium, nickel, zinc, and so on. Fude Li did an important contribution in this field, whose three published patents (patent Nos. CN93106616.6, CN96117479.X and WO9733837) refer to adsorption and reduction of heavy metals by sulfate reducing bacteria and other anaerobic bacteria and their application in electroplating wastewater and other wastewater containing metal ions. Besides sulfate reducing bacteria, scholars have also widely used bacteria, yeast, fungi and alga as the bio-adsorbent in recent years. For example, U.S. Pat. No. 5,055,402 proposed the use of dried dead alga to adsorb metal ions from wastewater. Moreover, U.S. Pat. No. 5,538,645 and U.S. Pat. No. 4,701,261 proposed adsorbing and recovering metal ions from wastewater by treated yeast. U.S. Pat. No. 4,690,894 proved that cells treated by lye could promote the adsorption effect of metal ions. A few patents raised the method that can improve metal ions adsorption using embedded microbial cells by hydrophilic material (U.S. Pat. No. 5,976,847 and CN02131031.9). Besides the patent literature above, a large number of general literatures refer to research of metal ions absorption of microbial cell.
Nevertheless, it is to be noted that these patents and general literatures mainly refer to non-growing cells of microbes that act the adsorbing material to adsorb metal ions, and the pretreatment of non-growing cells is complex, which limits adsorption efficiency. Meanwhile, literatures that relate to growing cells adsorbing metal ions are relative less.
Some microbes have the same adsorption function in their growing cells' metabolism. U.S. Pat. No. 6,383,388 describes a metal ion-resistant Saccharomyces cerevisiae which can adsorb and reduce chromium(VI) at low concentration (2 mg/L) to chromium(III) and adsorb such metal ions as molybdenum, nickel, zinc, calcium and cobalt in wastewater, in its growth. However, the patent did not mention the removal efficiency of other ions but that of chromium. U.S. Pat. No. 6,355,172, CN 1281524C and CN 1086366C proposed a technology of adsorbing metal ions by growing microbes attached on filter bed, in which wastewater continuously passes by filter bed, remove and wash off part of microbial cells in filter bed to recover metal ions adsorbed by the cells. By using microbes metabolism to form sulfides or metal oxides and hydroxides, the technology adopts a large amount of anaerobic microbes such as Shewanella, Desulfovibrio and Desulfococcus to deposit and recover metal ions. However, the patent did not mention the removal efficiency of metal ions.
Besides the electroplating industry, which is suitable for external biosorbent and removal of metal ions in anaerobic or anoxic environment because of its high concentration of metal ions in wastewater and little water, there are lots of other industries whose anaerobic conditions is difficult to control because of its varieties of metal ions, low concentration and large amount of water. Therefore, the biosorption under anaerobic condition is particularly important.